improve spark_connection by about 250ms by avoiding using SETX if variable is already set #102
improve spark_connection by about 250ms by avoiding using SETX if variable is already set #102

rstudio/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr